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SSTO Madness


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I have the worst case of "run way drift" for my bigger than life SSTO. I need help, the one time I was able to overcome the unexpected turning and reached the end my ship sinked slowly due to the weight, I added more engines since then but I can't reach the end of the runway as the plane continues to drift, it has been a day since I fixed the turning by moving the plane left and right. Here are some pictures of the plane.

 

file:///Users/2pattama/Desktop/Screen%20Shot%202016-09-21%20at%209.13.16%20PM.png

file:///Users/2pattama/Desktop/Screen%20Shot%202016-09-21%20at%209.14.43%20PM%202.png

file:///Users/2pattama/Desktop/Screen%20Shot%202016-09-21%20at%209.14.29%20PM.png

file:///Users/2pattama/Desktop/Screen%20Shot%202016-09-21%20at%209.14.05%20PM.png

 

PS how do u insert screenshots

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31 minutes ago, Mr. Failure Mcgee said:

PS how do u insert screenshots

Not like that, those are (broken) links to files on your local machine.
Upload your images to a file/image host of your choice (imgur / postimg etc), then use the "insert other media" button.

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This is just a guess, but I imagine you're too heavy for just the tricycle gear setup you've got going on there...your plane is sagging towards the tail because there's nothing holding it up, buckling your aft gear inwards from the vertical in the process, causing camber (if that's the right term) in the aft gear, which is steering your plane. @keptin discusses this phenomemon towards the bottom his Basic Aircraft Design Explained Simply, with Pictures thread.

TL, DR: Another couple of sets of gear aft towards the center of the plane and you should be fine.

Can't tell where your CoM is - could you add some screenies with your CoL and CoM turned on, purdy please? Given your engine configuration, I'd also suggest turning on CoT; you might have problems there.

Typically, if your plane sinks once you're airborne, it's either because you're excessively stable (i.e. CoL too far aft of the CoM) or you haven't got sufficient lift (i.e. your main wings are too small). Looking at your plane, I'ma guessing a combo of the two. Could also be insufficient pitch authority as well, I suppose. Really need to see those markers to be sure.

Edited by capi3101
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What @capi3101 suggests is very good advice, except I disagree that your craft has too small wings. Though, it does have too many engines, in my opinion.

The craft below weighs 290 t, but has it's main landing gear right below CoM (and ever-so-slightly behind) and therefore does not veer sideways.

It also has roughly the same wing area as yours and manages to takeoff easily and lift 100 t to orbit.

PoXFAOX.png

 

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Okay...couple of things I can see right away from your screenie.

Your CoL is definitely too far aft; you need to try to move it closer to the CoM if you can. As a first step, and if it wouldn't mess with the aesthetic you're going for with the design too much, I'd suggest some canards. They could be added to your plane with a minimal amount of fuss, they'd move the CoL forward and you'd gain some more pitch authority in the process. Alternatively, you can move the main wings forward on the plane a bit. @Val has already demonstrated that the wings will provide sufficient lift for the design.

The other main thing I'm seeing is the positioning of the aft gear - they should be aligned as close to the CoM as you can manage.

Both of these suggestions have to do with torque. See, you can think of your plane as a big lever along the three different axes (fore-to-aft, port-to-starboard, dorsal-to-ventral). The CoM acts as the fulcrum point in each case while in flight (while on the ground your aft wheels are doing the same thing). Your main wing and your control surfaces provide the effort necessary to change the plane's attitude, measured as the cross-product (one of those nasty mathematical things with which college physics professors like to torture poor suckers undergraduates) of the force vector with the distance from the fulcrum. Your main wings provide a torque, and the CoL is the point through which the force vector is acting. The further away that force vector is from the CoL, the greater the torque it provides. Now, you want some torque from the wings to keep your plane from flipping out of control, but you won't want so much that the control surfaces can't overcome the tendency of the main wings to push down the nose. That's why you want the CoL relatively close to the CoM.

Probably more to this explanation here, but I'm at work and today I will not be anywhere near a computer, so I'll let one of the other plane gurus take over/correct my ramblings.

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